1. Field of the Invention
This invention relates to a welding apparatus for welding flat webs or web sections made of synthetic thermoplastics, comprising two parallel welding bars, which are adapted to be raised and lowered relative to each other, and tensioned strips, preferably strips of which are reinforced by glass fiber woven fabrics, which strips cover said welding bars and are fixed to rolls, which are disposed beyond opposite ends of each welding bar and serve to wind up and unwind said tensioned strips.
2. Description of the Prior Art
Such welding apparatuses are known, e.g., from German Patent Specification 20 04 574, German Patent Publication 27 43 494 and Published German Applications 23 58 281 and 28 45 271. The tensioned strips which are made of polytetrafluoroethylene and cover the welding bars are used to prevent the welding bars from sticking to the plastic films. A welding apparatus of the kind described first hereinbefore and is known from practice will now be described with reference to FIG. 1 of the drawing. That known welding apparatus consists of the upper and lower welding jaws 2. A severing knife 3 is provided laterally of the upper welding jaw 2 and by drive means, not shown, is adapted to be lifted and lowered in unison with the upper welding jaw. The welding bars of the upper and lower welding jaws are covered by strips of polytetrafluoroethylene. The means for winding up and for unwinding the strips of polytetrafluoroethylene and for tensioning said strips over the welding bar are basically identical for the upper and lower welding jaws. For this reason such means will be described only with reference to the lower welding bar. The tensioned polytetrafluoroethylene strip which covers the welding bar has a width which is approximately as large as the length of the welding bar and is fixed to the rolls 5, 6. The polytetrafluoroethylene strip is unwound from the roll 5 and is wound up on the roll 6. The polytetrafluoroethylene strip is advanced in that it is wound up on the roll 6 and unwound from the roll 5. That operation is performed in dependence on the welding time and welding temperature. This means that the polytetrafluoroethylene strip is pulled forward by a predetermined length after a predetermined number of cycles so that an unused length of the polytetrafluoroethylene strip will be available between the welding bars for the succeeding operating time. The roll 6 for winding up the polytetrafluoroethylene strip is accommodated in a space 7, which is defined by the crossbeam 8, which carries the welding jaw, and by another crossbeam 9, which carries a retaining member for the web sections which have been severed, such as bags or sacks made of film. The retaining member consists of a spring-cushioned pad 10, which cooperates with a corresponding backing pad. The roll from which the polytetrafluoroethylene strip is unwound is mounted in a groove 12 that is disposed below the crossbeam 8 which carries the welding jaw. That roll is so designed that it will not laterally protrude beyond the outline of the welding jaw unit. Additional retaining members and structural elements are disposed on both sides of the welding jaw unit. Said additional elements act to sever and weld the film web during the standstill times and must be disposed as closely as possible to the welding and cutting region so that the film web will be held as taut as possible. As the rolls 5, 6 which serve to wind up and unwind the polytetrafluoroethylene strip must be disposed within the outline of the welding unit if a compact design is to be obtained, the known welding apparatus is highly expensive. Besides, the servicing of the known apparatus is complicated and time-consuming because the crossbeams 8, 9 must be removed before the rolls 5, 6 can be replaced when the polytetrafluoroethylene strip has been used up.
Another disadvantage of the known apparatus resides in that the rolls 5, 6 have been wound on long rods, which are usually relatively thin so that they may be deflected in such a manner that the tensioned polytetrafluoroethylene strip which covers the welding bars may become distorted and may form waves and creases.